LONDON—As one of Britain’s most senior counterterror investigators, former Detective Inspector Brian Tarpey conducted operations at the height of The Troubles in Northern Ireland; followed the trail of jihadis to North Africa; entered the notorious Black Beach prison in Equatorial Guinea; and tracked the 2005 London terrorists all the way back to their lawless training camps in Pakistan.

In all of his investigations, he says he was harmed only once: in Moscow, during the hunt for the killers of Alexander Litvinenko. The Russian dissident had died in London after being poisoned with a dose of the radioactive isotope polonium-210 that was slipped into a pot of tea.

The Russian authorities said they would help the team from Scotland Yard to run down their leads and interview the prime suspects. Instead the detectives encountered obstruction, ultimatums, subterfuge, intimidation, a possible body double, and even a case of suspected poisoning.

It was the most difficult foreign assignment of Tarpey’s career.

When he sat opposite a delegation from the Russian prosecutor general’s office at the beginning of the trip in December 2006, he had no idea that the rogues’ gallery on the other side of the long table featured some of Vladimir Putin’s top enforcers including a future member of the U.S. Treasury’s sanctions list and the suspected mastermind of Russia’s alleged pro-Trump influence campaign.

After just a few days of investigation in Moscow, it was obvious to Tarpey that this supposedly independent branch of the justice system was being run as a de facto intelligence operation with immense power and resources. They weren’t tested at the time, but he’s convinced that he and a colleague were given tainted cups of tea inside the prosecutor general’s office, which left them both suffering from gastroenteritis-like symptoms. Tarpey began to feel unwell as soon as he left the building.

“You’ve got professional assassins effectively running around south London cold-bloodedly murdering people and setting it up like suicide.”

The very first thing the prosecutors had demanded from the British delegation at the meeting was a quid pro quo—we’ll let you interview your witnesses if you help us secure access to an anti-Putin dissident who had sought exile in Britain. At each subsequent meeting they would ask for updates on their request to question the arch-critic Boris Berezovsky, an oligarch who was once a powerful ally of Vladimir Putin.

“It was a barter. They’d say ‘Yeah we’ll help you, but what about Berezovsky?’ And the long and short of it was that their investigator was allowed to come here to the U.K. and interview him,” Tarpey told The Daily Beast.

The British government has never admitted there was a trade-off to allow access to one of Putin’s most hated foes. Some five years after his visit from the Russians, Berezovsky was found dead with a ligature around his neck.

An inquest recorded an open verdict, with the coroner admitting that he did not know if this was a suicide or an unlawful killing. One of Berezovsky’s friends who most vociferously argued that the oligarch had been murdered was Nikolai Glushkov.

This week Glushkov body was found at his London home. He was the former head of the national airline Aeroflot, and was wanted in Russia on charges of fraud and embezzlement. His death may have looked like yet another suicide by a Russian dissident in England, but this time a pathologist observed that a compression to the back of the neck had been fatal.

Thus, Scotland Yard’s SO15 counter terror command opened its second inquiry into the murder or attempted murder of a Russian émigré in less than two weeks. Sergei Skripal and his daughter, Yulia, are still in the hospital after being poisoned with a Russian nerve agent in the city of Salisbury in south-west England.

“It just underlines the arrogance, doesn’t it,” Tarpey said. “In the midst of what’s happening in Salisbury, they may have gone and done another one. It’s almost like somebody is out of control. If you were the Mr. Big ordering these hits—which somebody obviously is—then you’d think, OK, maybe I’ll lay low a little while until Salisbury blows over before taking out the next one.

“For me the second one is almost as scary, if not more scary than the nerve agent. It looks like you’ve got professional assassins effectively running around south London cold-bloodedly murdering people and setting it up like suicide.”

The Home Office announced last week that it would review 14 cold cases reportedly linked to Russia by U.S. intelligence sources—many of them involved apparent suicide.

Either of these Russian murder inquiries could end with another Scotland Yard inquiry in Moscow. Investigators may be able to follow traces of the novichok nerve agent used on Skripal in a hunt that’s reminiscent of the radioactive breadcrumbs left in the Litvinenko murder.

The molecular signature of the chemical agent has already indicated that this substance was created in a Soviet-era covert chemical weapons laboratory.

If another team is dispatched to Russia, Tarpey thinks the detectives would be put at risk. “I have no reason to think they wouldn’t be,” he said. “They did it to us, and they’ve come over here and done this again.”

Late one winter’s night during the 2006 Moscow inquiry, Detective Inspector Tarpey—who is tall with broad shoulders and closely cropped hair—met his team back at the Radisson. “I remember looking around thinking there were three very distinct groups in the hotel bar; there were SO15 officers, FSB officers, and prostitutes,” he said. “All three were extremely obvious.”

Tarpey needed to brief Detective Chief Superintendent Timothy White on the day’s work out of earshot of the Russian security services, so they opted for a walk and talk looping around the hotel’s corridors.

Deep in conversation, the men discovered too late that one of those hallways was a dead end. As they spun around to head back in the opposite direction, a Russian man, maybe 25 yards behind them, froze like a rabbit in the headlights. “He did the worst thing that a surveillance officer could do,” said Tarpey.

Panicking, the undercover officer turned away and went for the first thing he could see, which was an elevator. He was still waiting for the doors to open when two of Scotland Yard’s finest strolled back past him. After watching the amateurish agent enter the elevator, Tarpey, a veteran of hundreds of surveillance shifts, said: “Tim, just stop here for a second.”

“They were introduced to a man lying in bed, totally covered in bandages.”

Sure enough, moments later the agent’s head popped back out of the elevator. He stared at the men in shock “like another rabbit in the headlights” before scurrying away down the corridor.

“I don’t know whether they thought we were messing them about,” said Tarpey. “He just wasn’t very good.”

Efforts by the prosecutor general’s office to stymie the operation ranged from ludicrous to outright obstruction of justice. When the British team presented the Russians with a list of their eight aims for the trip, which began with the most important tasks of interviewing witnesses Andrei Lugovoi and Dmitry Kovtun, they were told that this would indeed be possible but it would be better to start at number eight on the list and work up. It was a small, pointless intervention that showcased Russia’s desire to frustrate at every opportunity.

When the detectives arrived at Clinical Hospital No. 6 late one evening for an interview with Kovtun, which they weren’t allowed to record, they were introduced to a man lying in bed, totally covered in bandages save for his eyes and the bridge of his nose. “It could just as well have been you lying there,” Tarpey said. “My gut feeling is it probably wasn’t Kovtun. He’s never had a similar sort of profile as Lugovoi, who’s a real star out there. Without wanting to make it sound like a James Bond film, I suspect that Kovtun was probably a darker agent.”

The interview with this mysterious bandaged man—at which only one member of the British team was allowed—lasted for just 13 minutes before the authorities made their excuses and cut it short.

The inconveniences continued: Phones failed to work; film crews and photographers somehow knew where the British team was going to be next; and the tape of the interview with Lugovoi didn’t record. Scotland Yard only discovered that the Lugovoi tape was missing when they returned to London, it seemed their Russian hosts had forgotten to mention the recording mishap.

Tarpey said he had been outsmarted. “I like to think I’m fairly streetwise but I didn’t see that coming. Didn’t see it coming at all,” he said. “That was a low point in my career.”

The scale of deception employed to throw Scotland Yard off the scent makes more sense when you consider who the counter-terror officers were dealing with.

At that first meeting at the prosecutor general’s office, the men sitting behind the little row of Russian flags included Putin’s future chief of staff, Sergei Ivanov; Alexander Bastrykin, who is now on the U.S. Treasury’s sanctions list; and Saak Karapetyan, the deputy prosecutor general, who allegedly recruited a double agent inside Switzerland’s law enforcement agency who was supposed to be monitoring the Swiss bank accounts of Russian officials and oligarchs.

Ivanov, who has known Putin since their KGB days, is suspected of masterminding the Kremlin’s operation to influence the 2016 U.S. presidential election. The Senate Judiciary Committee demanded that Trump’s sons and Paul Manafort produce copies of any communications they had with him.

Bastrykin was the man chosen by Putin to destroy the domestic political opposition, according to The Atlantic, which reported on an extraordinary open letter published by the campaigning newspaper Novaya Gazeta when one of its reporters, Sergei Sokolov, was allegedly threatened with beheading by the head of the Investigative Committee:

Sokolov was placed in a car by your bodyguards. He was taken without any explanation to a forest near Moscow. There, you [Bastrykin] asked the bodyguards to leave you and remained face to face with Sokolov… The hard truth is that, in your emotional state, you rudely threatened the life of my journalist. And you joked that you would investigate the murder case personally.

At a separate meeting with the visiting British investigators, Yury Chaika, the prosecutor general himself, appeared. Chaika is a Putin loyalist with a string of corruption claims to his name—he is also the alleged source both of the Clinton kompromat offered to Donald Trump Jr. and the pro-Putin propaganda handed to controversial member of the House of Representatives, Dana Rohrabacher. He is also said to be a close friend of Natalia Veselnitskaya, who attended the infamous Trump Tower Russia summit.

A few days after meeting Tarpey to discuss his Litvinenko inquiry in Russia, The Daily Beast called the retired detective inspector with an update on the men who apparently had been interfering in his investigation.

“Wow, these are proper players,” he said. “They’re effectively Putin’s enforcers—gangsters with legitimate titles. They wouldn’t have thought twice about poisoning myself and Tim White with a bit of gastroenteritis then—they probably just did that to warm up.”

Tarpey knows better than most how dangerous the Russian regime can be, but even through his eyes, the scale of the threat is continuing to grow. He says he would think very differently if he was asked to pull together another Russia inquiry team to chase down the assassins who poisoned Skripal: “I think we’re into a new phase of crime with state sponsored terrorism.”

Last time round, he relied on some junior counterterror officers and civilians because of their Russian language skills. This time he would take a more experienced crew.

“I would never go voluntarily to Russia. I will not be going to the World Cup,” he said. “But if I was still a police officer, of course I would go back to Moscow.

“From my understanding of Salisbury, they’re a long way off sending anyone out there yet, but if they do, the first thing is be prepared for anything; double-check everything; don’t accept anything at face value, be prepared to argue your corner, be prepared to be messed around.”